In an engine/generator system, the fuel supply to the prime mover is directly controlled by an actuator means, generally with a throttle or injection pump. Fuel supply adjustment is a method of adjusting the torque/speed characteristics of the prime mover. As the throttle is closed or the fuel injection setting reduced, the torque produced at a given rotational velocity is similarly reduced, causing the system to slow down, while wide open throttle maximizes the torque and power output at a given speed. In conventional engine/generator systems, the speed of the system is controlled by altering the torque/speed characteristic of the prime mover. This results in engine inefficiency.
An additional cause of engine inefficiency is friction. As the piston moves through the cylinder, work must be done because of friction between the piston and the cylinder. Every time the piston moves through a stroke, some energy is lost, regardless of engine power output for that piston stroke. The greater the energy output per piston stroke, the less energy lost to friction as compared to energy output.
Other sources of engine inefficiency are parasitic loads such as oil pumps and the like, which vary with engine speed, but not power output. The higher the RPM of the engine, the more energy lost to these loads, regardless of actual engine power output. Parasitic loads and cylinder friction mean that for constant power output, lower RPM will generally be more efficient.
A partial solution for inefficiency during low demand is found in systems comprising an energy storage unit, such as a battery, in addition to the engine-generator system. An example for such system is the Hybrid Electric Vehicle (HEV), in which the solution for the inefficient low load mode is simply to turn the engine and generator off. The power demand is supplied by the battery until a higher demand is required or until the battery reaches a low energy level. The engine and the generator are then turned on to recharge the battery and to supply power, directly or via the battery, to the vehicle.
Attempts to deal with engine inefficiency, such as with the HEVs have not been successful. A drawback of HEVs is that a substantial amount of restarting of the engine and generator is required, which releases fumes and is a particularly inefficient period in engine operation. Secondly, when the engine is turned off, a great amount of inertial energy is lost. This results in a waste of energy, invested in regaining the inertial energy, when the engine is restarted. Third, no currently available energy storage system is perfect; more energy must be supplied to the energy storage system than is later removed as useable electricity, with the difference being lost as heat. Energy storage systems also have limited life, in terms of number of charge/discharge cycles, and aging due to deep discharge. Even in the hybrid electric vehicle, there is substantial utility to a wide range of generator power output, providing efficiency can be maintained over this range.
Methods for the conversion of electrical power at a given voltage, current, and frequency, to electrical power at a different voltage, current, or frequency are well known in the art. The simplest device, the transformer, is used to trade voltage for current with little loss of power to inefficiency.
Other devices convert input electrical power to an output electrical power via an intermediate mechanical form, or from alternating current to alternating current of different characteristics via intermediate direct current. Power electronics are devices that usually contain transistors or similar components, and use switches to vary the electrical characteristics of their output, according to requirements. These include many variations, some of which are the bipolar transistor, the darlington pair of transistors, the field effect transistor, the pulse width modulated DC controller, the Silicon Controlled Rectifier, the DC link converter, the insulated gate bipolar transistor, the MOS controlled thyristor, as well as optically driven devices, vacuum devices, gas filled devices, and even mechanical devices. Power electronic devices can often act as variable pseudo-resistance, that is they can create voltage/current output relationships without dissipating power in the fashion of an actual resistor. The Silicon Controlled Rectifier can control how much AC power is delivered to a load.
Electric control systems for generators are well known in the field of the art. Output of a desired frequency, voltage and current can either be achieved by controlling the operational state of the generator, or by converting the native output power of the generator to the desired voltage, frequency, current, or otherwise characterized output power.
Output characteristics of a generator are not independent, and are related by load considerations and generator internal characteristics. For example, a DC generator feeding a resistive load, when under circumstances that increase the output voltage of the generator, will also experience an increase in current flow. Often various changes in output are described with other output aspects held constant.
Systems involving an engine, for example a heat engine, providing output mechanical power for a load are common in the art. Often, between the engine and the load is a transmission, or mechanical advantage coupling, with a variety of possible gear ratios. The load may be a mechanical load, such as a set of vehicle wheels, or a generator, which converts rotary mechanical power into electricity. The output mechanical power of the engine is applied to the mechanical load after the torque/speed ratio of the mechanical power has been modified by the transmission.
Engine-generator systems, in which an engine is directly mechanically linked to a generator, and providing the generator with power in the forms of torque and speed, turn at a fixed speed relation. Torque supplied by the engine is not necessarily equal to the torque absorbed by the generator. If, due to some perturbation, the generator is unable to absorb all the torque that the prime mover provides, a potentially dangerous situation may arise, for the system is not operating in equilibrium. Usually, it is the speed of the engine, and with it, the speed of the generator, which will change when the system is not operating in equilibrium, and in the case mentioned above, the speed of the system will probably increase. Sometimes the fixed mechanical linkage between the engine and generator includes some sort of gearing or mechanical advantage. In this case, then when the system is operating in equilibrium, there will be an equilibrium between the individual linkages between motor and gearing mechanism, and between gearing mechanism and generator.
The term “torque load”, in the course of this specification, is used to mean the amount of torque that the generator absorbs from the engine or other prime mover, to which it is connected. It is also described as the torque in the direction counter to rotation that the generator applies to a transmission with the prime mover. A negative torque load would refer to a torque in the direction of rotation (i.e. the generator acting as a motor). The “torque of the generator” refers to the torque applied by the generator to the prime mover, in the direction of, or counter to, rotation. The prime mover in most cases may be an engine, but the term engine is used in the course of this specification to also refer to other prime movers that behave similarly to engines, with the characteristics that will be henceforth described.
An engine, supplying mechanical power to a load, such as a set of vehicle wheels, produces a torque, in the direction of rotation of a common shaft between the engine and the load. The load (in most cases) provides a resistive force to the same shaft, usually in the direction counter to rotation. This is termed the torque load of the mechanical load, and is usually a negative torque relative to the torque output of the engine. However, in the case where the mechanical load is a motor/generator, during the periods in which the motor/generator operates as a motor, it will provide torque to the shaft in the direction of rotation. This is also termed a torque load of the mechanical load, but it is a negative torque load. All forces other than those that originate with the engine, that have an effect on the torque of the output shaft of the engine, are termed the ‘torque load’. The torque load combines any parasitic loads present in the transmission with the mechanical output loads placed upon the transmission, as reflected by the gear ratio of the transmission.
It is the combined effect of the torque output of the engine together with the torque load that determines whether the shaft accelerates, decelerates or continues at a steady speed. When the shaft accelerates or decelerates, so does the unthrottled engine, and usually, so does the engine's combustion rate.
Each type of engine has a characteristic torque/speed relationship. Similarly, each transmission ratio has a characteristic torque/speed relationship. Also, each type of mechanical load has a characteristic torque load/speed relationship. Furthermore, the operating conditions may affect the torque/speed relationship of the engine, transmission, or mechanical load. The torque/speed characteristics of these components characterize their operating regime.
The torque/speed characteristics of engines, transmissions and mechanical loads, depend on control states and operating conditions. For example, throttling an engine changes the engine's torque/speed characteristic; changing the gear ratio changes the transmission's torque/speed characteristic; in the case where the mechanical load is an AC generator, then changing the frequency of the generator changes the mechanical load's torque-load/speed characteristic. Operating conditions that may affect the torque-load/speed relationship of the load include various gradients of an incline, in the example case where the mechanical load is vehicle wheels.
The rotational velocity at which the mechanical load receives the mechanical output of the engine is not necessarily equal to the rotational velocity the engine is outputting since this is first modified by the transmission. The term “transmission speed” as used herein means the speed which the mechanical load is provided with from the engine, but as reflected through the transmission. The transmission speed often affects the torque of the mechanical load.
The speed of the mechanical load is locked to the speed of the engine, although these are not necessarily identical, since a transmission interacts between them.
An unthrottled engine has the following characteristics:
When the torque load is equal to the engine output torque, there will be an equilibrium, and the unthrottled engine will maintain a steady speeds.
When the torque load is greater than the engine torque output, there will be a net torque on the engine side of the transmission that forces the unthrottled engine to decelerate.
When the torque load is less than the engine output torque, then there will be a net torque on the engine side of the transmission that forces the engine to accelerate.
As the unthrottled engine decelerates, its rate of combustion decreases, and its power output is reduced. As the unthrottled engine is forced to accelerate, its combustion rate increases and its power output is increased.
An example of how torque equilibrium, or lack thereof, can affect speed, is shown in starting an engine-induction generator system, in which the induction generator is supplying electricity to a fixed frequency, fixed voltage, electrical load. When an engine-generator system is started, the speeds of the engine and generator are in fixed relation to each other, but the torques of the individual parts of the system are not. The engine is producing a torque in the direction of rotation, and for equilibrium, the generator would have to be producing an equal torque in the direction counter to rotation. However, since the speed is so low, the generator does not yet generate electricity. In fact, due to the low speeds, the generator absorbs electricity (from another power source) and produces torque in the same direction as rotation. Therefore, the torque produced by the engine is not absorbed by the generator, resulting in system speed acceleration. However, as high enough speeds are reached, the generator begins to generate electricity, and to absorb the torque produced by the engine. There is (in a matched system) at least one equilibrium point, at which the torque output of the engine matches the torque absorbed by the generator, whereupon the system ceases acceleration, and a steady speed is maintained. When changes in system output are required, these are usually made by throttling the engine, or similar methods. Throttling acts to change the torque/speed characteristics of the engine. When throttling is used, the system will often change speed to a different torque equilibrium point, due to the new engine characteristics.
In all examples of engine-generator systems, each of the engine and the generator will have a characteristic that describes how its torque changes with speed. These characteristics will determine the equilibrium speed of the combined engine/generator system.
The generator torque/speed characteristics will depend upon the type of generator it is, its level of excitation, and the load to which it is supplying electricity. Induction generators, when attached to electrical loads having fixed voltage, fixed frequency characteristics, are known to be quite stable, being that within the speed range at which they generate electricity, they have a steep torque requirement relative to their speed requirement. Thus an accidental speed change in a system involving an induction generator will be immediately followed by a large torque change, which normally causes the system to return to equilibrium speed.
Note that in the pathological condition of the engine providing greater torque output than the maximum torque load of the generator, system speed will continue to increase until the internal friction of the engine prevents further speed increase, or until the system fails. This is known as a runaway condition. The maximum torque of an induction generator is limited by the load current. A sudden reduction in load will reduce the maximum torque load of the induction generator, and may cause a runaway condition.
In AC induction machines a method may be applied for the alternation between generator and motor modes. The synchronous speed, the speed of the rotating magnetic field, of the induction machine is determined by the number of poles of the machine and the frequency of the applied AC power. The synchronous speed is given by the formula: Ns=120 f/P where: Ns is the Synchronous speed in rotations per minute; f is the frequency of the power supply in cycles per second; and P is the number of poles for which the machine is is wound.
In induction generators the actual speed of the rotor is faster than the synchronous speed of the rotating field. In induction motors the actual speed of the rotor is lower than the synchronous speed of the rotating field. In fact, the motor and the generator are essentially the same machine with the main difference being in their actual speed in relation to their synchronous speed; induction machines that are marketed as motors are often placed into service as generators. Although a generator is supplying real electrical power to a load, it is consuming reactive power for excitation purposes. If an induction machine does not have a source of excitation power, then it will not develop a rotating magnetic field, and will not act as a generator.
By controlling the frequency of the input power to the excitation, a controller may increase or decrease the synchronous speed of the machine. For a given rotor RPM, the control may increase or decrease the synchronous speed to be faster or slower than the rotor RPM thereby determining the function of the induction machine as a generator or a motor.
U.S. Pat. No. 6,054,844 to Frank describes a system in which the torque applied by a motor/generator to the engine is calculated and applied to force the engine torque to have an ideal relationship to the calculated engine speed. Frank's methods of controlling the engine involve calculations and lookup tables as to how much torque the motor must force the engine to adopt so that the speed/torque relationship of the engine follows an ideal operating line. Another point Frank describes is how the engine is switched off if its speed is too low as to have non-ideal conditions. Furthermore, Frank's system is described only with reference to vehicles, and does not include other applications of engine-generator sets. Whilst the Frank apparatus controls engine power output via the torque/speed curve reflected from the wheels, with corrections for CVT dynamics terms, a need remains for a system that controls engine power output in a fashion which is independent of wheel loads.